Mesophase pitch is a carbonaceous material possessing an ordered liquid crystalline structure. It is believed that it is the liquid crystalline structure of mesophase pitch which lends superior physical properties to carbon artifacts, especially fibers, made therefrom.
Various processes for making mesophase pitch are well known in the art. One such process is disclosed in U.S. Pat. No. 4,208,267 (Diefendorf) wherein solvent fractionation is used to obtain a mesophase former fraction with a quinoline insolubles (QI) content of less than 25%. The Diefendorf patent teaches treating an isotropic pitch with a solvent to provide a solvent insoluble fraction having a sintering point below 350.degree. C. and separating the solvent insoluble fraction from the solvent solution. The solvent insoluble fraction is then heated to a temperature of from 230.degree.-400.degree. C. in order to convert the insoluble fraction to a deformable pitch containing greater than 75% mesophase.
U.S. Pat. No. 4,277,324 (Greenwood) teaches that QI and high softening temperature components are undesirable in a pitch intended for spinning and can be removed from a pitch by a solvent extraction process. In the Greenwood process a flux liquid (solvent) is added to the pitch to make the pitch fluid and suspend any insoluble pitch particles in the solution. The suspended particles are removed from the solution and the solution is treated with a second liquid (antisolvent) to cause a substantial portion of the fluid pitch to precipitate. The precipitated pitch is then removed from the fluid and used to make carbon artifacts.
U.S. Pat. No. 4,184,942 (Angier) teaches heat soaking of a low QI isotropic pitch to increase the amount of neomesophase former fraction in the pitch. After heat soaking, the neomesophase former fraction is isolated by solvent extraction.
Pat. No. 5,259,947 discloses a solvated mesophase pitch with improved handling characteristics. Because the solvated mesophase pitch melts at a temperature at least 40.degree. C. lower than the same pitch in non-solvated form, the solvated pitch may be successfully processed and spun at a lower temperature. Also, spinnable solvated pitches can be prepared from heavy aromatic pitches that are otherwise unsuitable, or that melt above 350.degree. C. where carbonizing occurs making these pitches unsuitable for spinning. Thus solvated mesophase effectively increases the range of heavy aromatics yielding a useable pitch. Because of this new utility of heavy pitch fractions, it is desirable to obtain such heavy fractions by an operable process having goo yields.
Typical heat soaking techniques (i.e., increasing the severity of the heat soak conditions) were tested in an attempt to increase the yield of heavy mesophase-forming aromatic pitch has some severe drawbacks. Although increasing the severity of the heat soak will usually increase the yield of heavy mesophase-forming aromatic pitch, the process will typically produce a two-phase product comprising an isotropic phase pitch and a mesophase pitch. Two phase pitch products are notoriously difficult to process, especially when a continuous processing system is employed where transfer of the two phase mixture is required. Typically, the two phases have disparate physical properties, e.g. molecular weight, viscosity, density, and melting point. During the heat soak process the mesophase portion tends to become very viscous, and eventually carbonizes inside the processing equipment, ultimately reaching a solid coke state. The viscous mesophase and/or solid coke will foul furnaces, transfer lines, associated vessels, and equipment. Even if the process is not taken this far, slight carbonization of the pitch is likely to introduce unmeltable contaminants in the mesophase pitch product. On the other hand, if heat soaking conditions are kept mild to avoid forming a two phase mesophase containing heat soaked pitch, the potential yield of heavy solvated mesogens is undesirably low. The present invention seeks to avoid the processing problems as outlined above and in addition, improve the yield of the desired mesophase pitch product.